Fancy Goldfish

Carassius auratus
Goldfish are natural born pigs. They are messy. And they eat a lot. Thus creating the biggest obstacle for many goldfish keepers - the water quality. Like most fish, goldfish prefer to live in clean water. They can tolerate a wide range of pH, but prefer neutral to slightly basic water. They do not do well in any aquarium with ammonia or nitrite present. Since they can easily produce huge quantities of wastes, and ammonia, they should be kept in well-filtered aquariums. That's right-filtered aquariums. Problem is most people try them in a very small bowl first. Setting up a goldfish in a 4 liter bowl is a recipe for disaster. As I mentioned, they eat a lot. They also produce a lot of waste. So in this small container, they will accumulate very high concentrations of the chemicals most toxic to them. You could do daily water changes, but this can be stressful to the fish. So it's best to house the goldfish in a filtered aquarium.
Since goldfish are so messy, and will grow rather large- it's best to house only one small fish per ten gallons. As they get larger, each fish could easily take up thirty gallons each! This and the fact they tend to root around the gravel and disrupt plants and rock work, and the fact they like cooler temperatures, makes them less than ideal for the community aquarium. In general, goldfish are peaceful fish and will get along well with a variety of other species, but they have different needs than most other fish and most likely will never thrive in the community aquarium.
Too many aquarists have killed their goldfish with kindness trying to appease that bottomless appetite. Unfortunately, it is quite possible for that fish to eat itself to death. To literally tear its intestines, or create a backlog of digestion leading to death. So the simple solution is to monitor feeding. Feed them one or two pellets per day.
Goldfish come in many shapes and sizes. They live over 20 years, so think carefully will you be able to keep this big fish for that long.
Photo by Dusko Bojic